How can I scale an entire web page with CSS?

You might be able to use the CSS zoom property – supported in IE 5.5+, Opera, and Safari 4, and Chrome

Can I use: css Zoom

Firefox is the only major browser that does not support Zoom (bugzilla item here) but you could use the “proprietary” -moz-transform property in Firefox 3.5.

So you could use:

div.zoomed { 
    zoom: 3; 
    -moz-transform: scale(3); 
    -moz-transform-origin: 0 0;
}